An understanding of cellular, biochemical and molecular mechanisms that serve physiologically and pharmacologically to regulate myometrial function at the contractile element level is essential for understanding the cause of labor and development of strategies to prevent preterm birth. Phosphorylation of myosin light chain by Ca2+/calmodulin- dependent myosin light chain kinase initiates smooth muscle contraction whereas dephosphorylation by protein phosphatase induces relaxation. In addition to Ca2+, there are other second messengers including cAMP and diacylglycerol that may alter myometrial contractility by activation of specific protein kinases. Rat myometrial tissue and primary smooth muscle cells in culture will be used to examine directly the effects of pregnancy and preparedness for labor on the biomolecular processes that regulate contractility and responsiveness by these second messenger systems. The effects of prostaglandins (PGE2a, and PGI2), uterotonic peptides (oxytocin, endothelin and effectors of protein kinases (isoproterenol and phorbol 12, 12 dibutyrate) on the mechanical properties of contraction will be established in relation to extents and sites of phosphorylation of myosin light chains, myosin heavy chains, caldesmon and myosin light chain kinase. Enzymatic activities, protein content, isoforms and mRNA levels for key contractile proteins will be characterized in myometrial tissues from pregnant and steroid treated animals. The direct effects of estrogen and progesterone on biochemical indices of contractile responsiveness will be evaluated in myometrial smooth muscle cells in culture. Enzyme activities, protein content, mRNA levels, and isoforms of proteins in the contractile element system will be measured. Responses to prostaglandins and other uterotonic agents, and changes in responsiveness resulting from steroids will be examined in relation to cytosolic Ca2+ levels, and the extents and sites of phosphorylation of myosin light chains, myosin heavy chains, caldesmon and myosin light chain kinase. Finally, the effect of controlled, cyclic tension on myometrial smooth muscle cells in culture will be examined as a more appropriate cell culture model system in comparison to the standard model of growing cells on a rigid surface. These investigations will provide fundamental information on myometrial function, and biomolecular changes associated with pregnancy and parturition.